This invention relates to radiation curable coating compositions for superconductors. More particularly, this invention relates to radiation curable coating compositions for superconducting wires that comprise a (meth)acrylate terminated urethane oligomer, an adhesion promoter and a (meth)acrylate reactive diluent.
Some materials when cooled below a certain critical temperature (Tc) become superconducting. At the particular Tc of the material, the electrons in the material that are responsible for conduction undergo a collective transition to an ordered state having many unique and remarkable properties. These properties include the loss of resistance to the flow of an electrical current, the appearance of unusual magnetic effects such as a large diamagnetism, substantial alteration of thermal properties and the occurrence of quantum effects otherwise observable only at the atomic and subatomic level.
Some twenty six of the metallic elements are know to be superconductors in their normal forms and another ten become superconductive under pressure or when prepared in the form of highly disordered thin films. Typically, such materials become superconductive only at very low temperatures, such as the boiling point of liquid helium, which is 4.2K. These superconductors have been called low temperature superconductors. However, it has recently been discovered that sintered materials comprising oxides of the elements of group IIa or IIIa of the periodic table can act as superconductors at higher temperatures, such as the temperature of boiling liquid nitrogen (77K). Superconductors based on such materials have been called high temperature superconductors.
There are many potential applications for superconductors, including, but not limited to, magnets for high energy physics applications, rotating machinery (i.e., synchronous generators, homopolar d-c machines), fusion magnets, magnetodynamic generators and magnets for nuclear magnetic resonance imaging, which is also called magnetic resonance imaging. Other applications include motors for marine propulsion and levitated trains for high speed transportation.
To effectively use superconductors in certain applications, superconducting wires must be made. Typically, superconducting wires are made with a metal sheath surrounding a superconducting core. Once a superconducting wire has been made, it is desirable to coat the wire with a dielectric composition. The coating, in addition to providing better structural integrity and protection from environmental stress, insulates wires from each other, particularly when wires are used in windings for motors, magnets and the like.
A coating for a superconducting wire must, however, possess certain properties. For example, the coating must be easy to apply and cure. Preferably, the coating and cure are carried out at ambient temperature. The coating composition should produce or contain a minimum amount of volatile organic compounds that may be emitted into the atmosphere, and the coating composition should be amenable to high speed production operations, and thus have high cure speeds. Lastly, the coating must be able to withstand the temperatures to which the superconducting wire will be subjected. Typically, a superconducting wire is cycled from ambient temperature to the Tc of the superconductor by introducing the superconductor into an environment having the temperature of the boiling point of liquid helium if the superconductor is a low temperature superconductor or the temperature of the boiling point of liquid nitrogen if the superconductor is a high temperature superconductor. Because the difference between the ambient temperature and the Tc is generally very large, the coating must be able to withstand such thermal cycling without detaching from the wire, cracking, splitting or failing in any other way that would affect the insulative or protective functions of the coating.
Thermal cycling can generate mechanical stress in coating compositions because of the differences in thermal expansion between the coating and the metal sheath and superconductor, and most organic polymers, the primary components of many coating compositions, are very brittle at the critical temperatures of both high and low temperature superconductors.
The ability of a coating to retain its integrity during such thermal cycling has been a nemesis to researchers attempting to find suitable coating compositions for superconducting wire. Further, thermally cured coating compositions generally are not preferred for high temperature superconductors because the temperatures required to cure the coatings can reduce the amount of current that can be carried in a wire before losing its superconductive properties. Thus, it is preferable to avoid heating superconductors.
The present invention provides an organic coating composition for superconducting wire that can be cured at ambient temperature and which will withstand the thermal cycling process that is necessary to reach the superconductor""s critical temperature.
The present invention provides a free radical ultraviolet light radiation curable coating composition for superconducting wire that comprises at least one (meth)acrylate terminated urethane oligomer; at least one (meth)acrylate functional acidic adhesion promoter; at least one (meth)acrylate functionalized reactive diluent; and at least one free radical photoinitiator. The (meth)acrylate terminated urethane oligomer may be made from a polyol, a polyisocyanate, and a hydroxy functional (meth)acrylate compound.
The present coating compositions are radiation curable, particularly UV curable and are able to withstand repeated thermal cycling from ambient temperature to the critical temperature of the superconducting wire.
This invention provides radiation curable coating compositions for superconductors. Preferably, the superconductors are in the shape of a wire. A superconducting wire has two major components: a superconducting core and a metal sheath that surrounds the superconducting core. In the case of low temperature superconducting wires, the superconducting core typically is composed of one of some 26 metallic elements in their normal form or alloys of certain metallic elements, which become superconductive above the boiling temperature of liquid helium. Metal sheaths for superconductors include, but are not limited to, copper, silver, iron, tin, aluminum, nickel, chromium, palladium, platinum, and gold and alloys thereof. The metal sheath of a low temperature superconducting wire is preferably made of copper. In contrast, high temperature superconducting wire is typically composed of a ceramic core of sintered oxides of elements of group IIa or IIIa of the periodic table such as (La, Ba)2CuO4, (La, Sr)2CuO4 and Baxe2x80x94Yxe2x80x94Cu type, and the metal sheath is preferably silver. It is contemplated that the present coating composition would be suitable for application to any type of material that shows superconductive properties and for which the coating has no detrimental effect on the superconductor. Thus, both low and high temperature superconductors of any composition are within the scope of the invention.
A coating for a superconducting wire is applied to the wire, including the core and sheath, and then cured. Ordinarily, a superconducting wire is coated with only one coating. However, it is possible to coat a superconducting wire with more than one coating having the same or different compositions. If the superconducting wire has more than one coating, each coating may be applied and then the coatings cured at once, or each coating or group of coatings can be applied and then cured followed by the application of another coating or group of coatings until the desired number of coatings has been applied. It is also contemplated that a coating composition may be used to bundle a group of superconducting wires. In other words, two or more superconducting wires may be bonded to each other by the application of a coating composition that is cured subsequent to application. The coating compositions of the present invention may also be used to seal a coil or winding of superconductor wire.
The present coating compositions may be radiation cured. Preferably, the curing is carried out by irradiating the coating with ultraviolet light. However, coating compositions can also be cured by irradiating the composition with a electron beam. Generally, any method that will begin a free radical polymerization of the components of the coating may be used to cure the coating. However, thermal curing is not preferred in most cases as the advantageous properties of most high temperature superconductors are adversely affected by heating. Thus, it is preferable to cure coating compositions at ambient temperature or below. As used herein, the term xe2x80x9cambient temperaturexe2x80x9d shall mean about room temperature.
When a composition is cured using ultraviolet light, it is preferable to include a photoinitiator in the composition. The photoinitiator is generally present in an amount ranging from about 0.1 to about 10 weight % of the composition. Examples of suitable photoinitiators include, but are not limited to vicinal ketaldonyl compounds (i.e., compounds containing a ketone group and an aldehyde group), 2,2-dimethoxy-2-phenyl acetophenone, xanthone, fluorenone, benzophenone, camphorquinone, anthraquinone, bisimidazole, perfluorinated diphenyl titanocene, 3-methyl acetophenone, 4-chloro-benzophenone, 4,4xe2x80x2-dimethoxy benzophenone, 4,4xe2x80x2-diaminobenzophenone, Michler""s ketone, benzoin propyl ether, benzoin ethyl ether, benzyldimethylketal, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropane-1-one, p,p-bis[(triphenylphosphono)methyl]benzophenone salt, thioxanthone, diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2-methyl-1-(4-(methylthio)phenyl)-2-morpholinopropane-1one, 1-hydroxycyclohexylphenyl ketone (IRGACURE 184 manufactured by Ciba Geigy), diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (Lucirin TPO manufactured by BASF), 2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173 manufactured by Ciba Geigy) and the like.
It may also be preferable to include in the composition along with the photoinitiator an agent such as triethyl amine, diethyl amine, N-methyldiethanolamine, ethanolamine, 4-dimethylaminobenzoic acid, 4-dimethylamino-isomethylbenzoate, 4-dimethylaminomethylbenzoate, 4-dimethylaminoisoamylbenzoate and the like to enhance the conversion of photoabsorbed energy to polymerization initiating free radicals.
The free radical ultraviolet light curable coatings of the present invention comprise a (meth)acrylate terminated urethane oligomer, a (meth)acrylate functionalized acidic adhesion promoter and a (meth)acrylate reactive diluent. As used herein the term xe2x80x9c(meth)acrylatexe2x80x9d, and the various grammatical forms thereof, shall mean esters that are the reaction product of acrylic or methacrylic acid with a compound that contains a hydroxyl functional group.
The (meth)acrylate terminated urethane oligomer can be made, for example, by first reacting a polyol with a polyisocyanate. As used herein the term xe2x80x9cpolyolxe2x80x9d shall mean a compound having two or more hydroxyl functional groups, including, but not limited to, polyalcohols, polyethers, polyesters, polycarbonates, glycerol, glycols, pentaerythritol, phenols, polyglycols, alkyds and glucosides.
Representative of the polyesters are the reaction products of polycarboxylic acids or their anhydrides and polyols, preferably diols. Suitable polycarboxylic acids and anhydrides include phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, tetrahydrophthalic acid, hexahydrophthalic acid, sebacic acid, succinic acid, glutaric acid, malonic acid, pimelic acid, suberic acid, 2,2-dimethylsuccinic acid, 3,3-dimethylglutaric acid, 2,2-dimethylglutaric acid and the like, and anhydrides thereof and mixtures thereof. Suitable polyols include 1,4-butanediol, 1,8-octanediol, trimethylol propane, pentaerythritol and the like.
Representative of the polycarbonates are polyhexamethylene carbonate and polycyclohexane dimethylene carbonate.
Representative of the polyethers are polyalkylene oxide, alkyl substituted polyalkylene oxides, alkyl substituted poly(tetrahydrofurans), and copolymers of the alkyl substituted tetrahydrofurans. and a cyclic ether.
Representative of the polyalkylene oxides are poly(propylene oxide), commercially available from Arco Chemical under the trade designation Arcol PPG 1025 and poly(tetramethylene glycol), commercially available from DuPont under the trade designation Terathane 1000.
As used herein the term xe2x80x9cpolyisocyanatexe2x80x9d shall mean a compound having two or more isocyanate functional groups. Diisocyanates are preferred. Examples of diisocyanates include, but are not limited to, toluene diisocyanate, isophorone diisocyanate, trimethylhexane diisocyanate, dicyclohexylmethane diisocyanate, diphenylmethane diisocyanate, phenylene diisocyanate and diphenyl diisocyanate.
Typically, a polyol is reacted with a polyisocyanate to form a urethane oligomer having reactive isocyanate functional groups. The urethane oligomers having reactive isocyanate functional groups may then be reacted with a (meth)acrylate compound having a hydroxyl reactive functional group, to yield a (meth)acrylate terminated urethane oligomer (i.e., end-capped urethane oligomer).
An end-capped urethane oligomer can be made in many ways. For example, an end-capped urethane oligomer may be made by reacting a diisocyanate with a capping agent to form a monoisocyanate, which is then reacted with a polyol to form the end-capped urethane oligomer. Alternatively, a polyol, polyisocyanate and capping agent may be reacted together at once, or a polyol and polyisocyanate may be reacted first and the resulting urethane oligomer end-capped. Preferably, the capping agent is a monohydroxy (meth)acrylate such as hydroxymethyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate or hydroxybutyl acrylate and the like. Most preferably, the monohydroxy functional acrylate is 2-hydroxyethyl acrylate. The end-capped urethane oligomers may be found in the composition in an amount in the range of about 10 to about 80% by weight of the total composition.
The composition also contains (meth)acrylate reactive diluents. The reactive diluents can be a monoacrylate, i.e. having only one acrylate group, or the reactive diluents can have more than one acrylate group such as trimethylolpropane triacrylate. Preferred reactive diluents include, but are not limited to, tetrahydrofuryl acrylate, ethoxyethyl acrylate, trimethylpropane ethoxylate triacrylate, methoxyethyl acrylate, hydroxy functional caprolactone acrylate, ethylene dimethacrylate, diethylene glycol dimethacrylate, diethylene glycol diacrylate, triethylene glycol dimethacrylate, triethylene glycol diacrylate, tetraethylene glycol dimethacrylate, tetraethylene glycol diacrylate, polyethylene glycol dimethacrylate, polyethylene glycol diacrylate, diethylaminoethyl methacrylate, glycidyl methacrylate, epoxy acrylate, glycidyl acrylate, 2-hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, hydroxypropyl acrylate, hydroxybutyl methacrylate, hydroxybutyl acrylate, hydroxyhexyl methacrylate, hydroxyhexyl acrylate, butanediol dimethyacrylate, butanediol diacrylate, propanediol dimethacrylate, propanediol diacrylate, pentanediol dimethacrylate, pentanediol diacrylate, hexanediol dimethacrylate, hexanediol diacrylate, neopentylglycol dimethacrylate, neopentylglycol diacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, trimethylolethane triacrylate, trimethylolethane trimethacrylate, polypropylene glycol diacrylate, polypropylene glycol dimethacrylate, glyceral monomethacrylate, acrylic acid, methacrylic acid, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, pentyl methacrylate, hexyl methacrylate, octyl methacrylate, lauryl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, stearyl methacrylate, methyl acrylate, butyl acrylate, ethyl acrylate, and the like. The (meth)acrylates are generally present in an amount from about 10 to about 75 weight % of the entire composition. The monoacrylates can be found in the composition in an amount in the range of about 10 to about 60% by weight of the entire composition. Compounds having more than one acrylate group may be found in the composition in an amount in the range of about 0 to about 75% by weight of the entire composition, and the composition may contain both monoacrylates and acrylates having more than one acrylate group.
The composition also contains an adhesion promoter. The adhesion promoter is preferably a (meth)acrylate functionalized carboxylic acid or phosphoric acid. Some examples of suitable adhesion promoters include, but are not limited to, acrylic acid, methacrylic acid, xcex2-carboxyethyl acrylate, Ebecryl 170 and Ebercyl 169. The Ebercyl products are acrylate ester derivatives, available from Radcure Specialties in Atlanta, Ga., and are phosphoric acid based adhesion promoters.
Mono or diesters of phosphoric acid having the following formula are also suitable adhesion promoters: 
where
m+1+p=3
R=H or CH3 
A=CnH2n, and 2xe2x89xa6nxe2x89xa66
Rxe2x80x2=C1to C14 alkyl, aryl, alkaryl, or alkyleneoxy
Representaive of the various species of organo-phosphate esters having the above formula include, but are not limited to, (1) methylmethacryloyloxyethyl phosphate (R=CH3; A=xe2x80x94C2H4xe2x80x94; Rxe2x80x2=CH3, m, 1 and p=1); (2) ethyl methacryloyloxyethyl phosphate (R=CH3; A=xe2x80x94C2H4xe2x80x94; Rxe2x80x2=C2H5); m, 1 and p=1); (3) propylacryloyloxyethyl phosphate (R=H; A=xe2x80x94C2H4xe2x80x94; Rxe2x80x2=C3H7; m, 1 and p=1); (4) methyl acryloyloxyethylphosphate (R=H, A=xe2x80x94C2H4xe2x80x94; Rxe2x80x2=CH3, m, 1 and p=1); (5) ethylacrlyoyloxyethylphosphate (R=H; A=xe2x80x94C2H4xe2x80x94; m, 1 and p=1; Rxe2x80x2=C2H5); (6) propyl methacryloyloxyethylphosphate (R=CH3; A=xe2x80x94C2H4xe2x80x94; Rxe2x80x2=C3H7; m, 1 and p=1); (7) bis(methacryloxyethyl) phosphate (R=CH3; A=xe2x80x94C2H4xe2x80x94; m=2; 1=0; p=1); and (8) bis(acryloxyethyl)phosphate (R=H; A=xe2x80x94C2H4xe2x80x94; m=2; 1=0; p=1).
The adhesion promoter helps the coating composition adhere to the superconducting wire, and, in particular, to the metal sheath. The adhesion promoter may be used in an amount in the range of about 1 to about 30% of the composition.
In addition to the above components, the composition may also contain other components that are known to those skilled in the art including stabilizers, pigments, surfactants, plasticizers, chain transfer agents and the like.
It was surprising and unexpected to find that the combination of the (meth)acrylate urethane oligomer, (meth)acrylate monomer, and adhesion promotor gave coatings that survived thermal cycling to 77K on superconducting wire.